The present invention relates generally to the art of locating a blood vessel lesion in a human subject, and more particularly, to an apparatus and method to efficiently identify a lesion and grade the stenosis using magnetic resonance imaging (MRI) technology.
The narrowing or constriction of vessels carrying blood to the heart is a well-known cause of heart attacks, and gone untreated, can lead to sudden death. In such stenotic vessels, it is known that the flow in the vessel at the point of narrowing and immediately after the narrowing is characterized by rapid flow velocities and/or complex flow patterns. In general, narrowing of blood carrying vessels supplying an organ will ultimately lead to compromised function of the organ in question, at best, and organ failure at worst. Quantitative flow-velocity data can readily aid in the diagnosis and management of patients and also help in the basic understanding of disease processes. There are many techniques available for the measurement of regional blood flow to a specific region of the anatomy, including imaging based methods using radiographic imaging of contrast agents, both in projection and computed tomography (CT), ultrasound, and nuclear medicine techniques. Radiographic and nuclear medicine techniques require the use of ionizing radiation and/or contrast agents. However, none of these techniques provide instantaneous flow-velocity measurements at a specific spatial location and/or specific time in the cardiac cycle. Two methods that are in current use are doppler ultrasound using an external transducer or the more invasive method of an intra-vascular doppler ultrasound guide-wire/probe.
The functional significance of a stenosis is conventionally determined using Doppler ultrasound to measure the velocity/pressure gradient across the vessel constriction along the axis of flow. The higher the gradient, the more significant the stenosis. However, using Doppler ultrasound is dependent on having an acoustic window allowing the ultrasound beam to insonify the vessel of interest at an angle of incidence as close to zero (i.e., parallel to the vessel) as possible. Furthermore, Doppler ultrasound does not provide the quality of images that are produced using MR technology. Further, ultrasound techniques are difficult to apply in certain situations because of intervening tissues such as bone, excessive fat or air. The use of an intra-vascular doppler ultrasound probe avoids some of these pitfalls but the procedure is quite invasive and has an associated risk of patient morbidity.
Phase contrast magnetic resonance angiography (MRA) is a practical and clinically applicable technique for imaging blood flow-velocities. MRI utilizes radio frequency pulses and magnetic field gradients applied to a subject in a strong magnetic field to produce viewable images. When a substance containing nuclei with net nuclear magnetic moment, such as the protons in human tissue, is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field (assumed to be in the z-direction), but precess about the direction of this magnetic field at a characteristic frequency known as the Larmor frequency. If the substance, or tissue, is subjected to a time-varying magnetic field (excitation field B1) applied at a frequency equal to the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be nutated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated (as the excited spins decays to the ground state) and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Phase contrast MRA makes use of flow encoding gradient pulses which impart a velocity-dependent phase shift to the transverse magnetization of moving spins while leaving stationary spins unaffected (Moran P. R. A Flow Velocity Zeugmatographic Interlace for NMR Imaging in Humans. Magnetic Resonance Imaging 1982; 1: 197-203). Each phase contrast acquisition generates two images: a magnitude image that is proportional to the proton density of the object and may also be T1-weighted, and an image representing the phase of the object. The phase image produced has information only from the moving spins and the signal from stationary tissue is suppressed. Images representing both the average flow-velocity over the entire cardiac cycle and at a series of individual points in the cycle have been generated using this technique. The phase contrast MR method produces phase images with intensities that represent the magnitude of the flow velocity and also the direction of flow. Therefore, such images may be used for both qualitative observation of blood flow and quantitative measurement. The practical application of phase contrast MR angiography and venography to the quantitative determination of flow velocity is therefore evident.
It would also be advantageous to use magnetic resonance imaging technology to efficiently locate and identify a stenosis in a blood vessel and use this MR technology to grade the stenosis for patient management decisions. Previous attempts at using MR technology to improve the ability to detect and grade coronary artery stenosis, for example, have relied primarily on using a single scan and decreasing the intra-voxel flow dephasing effects by decreasing pixel size, together with using first moment gradient nulling for flow compensation, and decreasing echo time (TE). It would be desirable to improve on this prior art by accomplishing the converse. That is, it would be advantageous to increase the intra-voxel flow dephasing effects to exacerbate flow voids, and therefore increase the conspicuity of lesions on the coronary artery that result in a stenosis in a quick screening exam. It would also be advantageous to have a method and apparatus for efficient visualization of a stenosis using MR technology followed with a more thorough exam if a stenosis is detected initially.
The present invention relates to a method and apparatus for efficient stenosis identification and assessment using MR technology, that solves the aforementioned problems.
The present invention includes a two step approach to accurately identify a blood vessel lesion and specify the degree of stenosis. In the initial step, an examination for lesion identification is disclosed using a low spatial resolution MR image. Preferably, the MR image is acquired using a gradient echo imaging pulse sequence with a flow sensitive bi-polar gradient waveform. The bi-polar gradients generate a broad distribution of velocities in a large voxel. Since a stenosis present in a given voxel will result in intra-voxel flow dephasing in voxels immediate to and distal to the stenosis, the stenosis can be quickly and efficiently localized using the initial step. After the stenosis is identified, a second step is performed in which a high spatial resolution MR image is acquired for more accurate and specific grading of the stenosis in the targeted area.
According to one aspect of the invention, a method of identifying a stenotic vessel using MR imaging is disclosed which includes performing a screening study by acquiring a first MR image having a low resolution to scan a suspected stenosis region. The method next includes analyzing the first MR image to identify a suspected stenosis within the suspected stenosis region, then performing a detailed study by acquiring a second MR image having a higher resolution than the first MR image, to scan the identified suspected stenosis. Next, the second MR image is analyzed to identify and/or grade an actual stenosis.
In accordance with another aspect of the invention, an examination method is disclosed to identify a lesion in a blood vessel and grade a stenosis resulting therefrom. The examination includes acquiring a first MR image using a gradient echo imaging pulse sequence having a flow sensitizing bi-polar gradient waveform, and detecting and localizing a suspected stenosis using the first MR image. The method next involves acquiring a second MR image if a stenosis is detected and localized. The second MR image has a higher resolution than the first MR image and is acquired in a region in which the suspected stenosis is detected and localized to grade the suspected stenosis. If a stenosis is not detected and localized, the examination is ended without further MR image acquisitions.
In accordance with another aspect of the invention, an MRI apparatus is disclosed to conduct MR stenosis screening, and if necessary, grade a stenotic vessel that includes an MRI system having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field, an RF transceiver system, and an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly in order to acquire MR images. The MRI apparatus also includes a computer programmed to operate the MRI system in two modes of operation to efficiently conduct a stenosis exam. The first mode is programmed to acquire at least one first MR image with low resolution over a relatively large region, then allow a user to analyze the at least one first MR image for an indication of a stenosis. The first mode of operation also includes receiving input to either end the stenosis exam or switch to the second mode of operation. In the second mode of operation, the computer is programmed to create a localized region of the relatively large region in order to target a suspected stenosis, and then acquire at least one second MR image with resolution higher than that of the at least one first MR image of the localized region.
In accordance with yet another aspect of the invention, the aforementioned methods are implemented in a computer program that is fixed on a computer readable storage medium that, when executed, causes the computer to acquire a first MR image of a relatively large region. The first MR image has high phase cancellation/intra-voxel dephasing in the immediate vicinity of a stenosis to screen a patient for possible arterial lesions. The computer is further programmed to limit a field-of-view (FOV) to a target region within the relatively large region if a possible arterial lesion is identified, and then acquire a second MR image of the targeted region. The second MR image having a resolution higher than that of the first MR image, and only being acquired if the first MR image indicates the presence of a lesion or stenosis.
In this manner, the higher resolution targeted acquisition near the site of interest is performed only if a lesion is present to effectively grade the stenosis. This technique provides a two-step technique involving a first step with increased sensitivity to detect lesions that can be acquired quickly, and then only performing the more time-consuming second step of acquiring an image with high specificity for grading the lesion only if one is detected in the first step. This two-tiered approach increases the efficiency for accurate coronary artery stenosis detection and assessment.